A novel salinity proxy based on Na incorporation into foraminiferal calcite

Salinity and temperature determine seawater density and differences in both thereby control global themohaline circulation. Whereas numerous proxies have been calibrated and applied to reconstruct temperature, a direct and independent proxy for salinity is still missing. Ideally, a new proxy for salinity should target one of the direct constituents of dissolved salt, such as [Na+] or [Cl ]. This study investigates the impact of salinity on foraminiferal Na/Ca values by laser ablation ICP-MS analyzes of specimens of the benthic foraminifer Ammonia tepida cultured at a range of salinities (30.0–38.6). Foraminifera at lower salinities (30.0 and 32.5) added more chambers (10–11) to their test over the course of the experiment than foraminifera cultured under higher salinity (36.1, 7–8 chambers, and 38.6, 6–7 chambers), suggesting that lower salinity promotes growth rates in this species. The Na/Ca of cultured specimens correlates significantly with seawater salinity (Na/Ca = 0.22S 0.75, R2 = 0.96, p < 0.01) and size. Values for Na/Ca and DNa vary between 5.17 and 9.29 mmol mol 1 and 0.12–0.16 × 10 3, which is similar to inorganic precipitated calcite. The significant correlation between test size and Na/Ca results from co-variation with salinity. This implies that foraminiferal Na/Ca may well be a robust and independent proxy for salinity, enabling independent salinity reconstruction. The quantified effect of salinity on Mg/Ca in our culture experiment, furthermore allows a direct correction for the bias in Mg/Ca based temperature reconstructions caused by differences in salinity.